Aso day trip?

I'm flying to oita next week. I will spend a while in beppu and oita before heading towards Kumamoto. On route I want to visit mount aso.
I wonder, is it feasible to do aso as a day trip and make it to Kumamoto at the end of the day?
How long does it to actually get up the mountain and see what there is to see from the station?

How do you want to travel - are you driving? Or taking a bus or a train? For the latter 2 you can get a pass, either a Sun Q Pass or a Kyushu Rail Pass (if you will be continuing on your journey). The express train will save you about 90 minutes in getting to Aso.
Beppu is a nice city to enjoy the hot springs, Hells Tour, monkey park/aquarium, and Mt. Tsurumi. You can see some previews at:
https://www.youtube.com/watch?v=K2NNi8MLcXM
https://www.youtube.com/watch?v=zepPfkuqhok
https://www.youtube.com/watch?v=cBnZneDsKqQ
Needless to say there is no guarantee you will be allowed to see Aso. If the poison gases coming out are too strong, the area near the crater will be closed off. It'll take about half an hour by bus from Aso Stn to the crater.
From Aso you can continue on the JR Hohi Line to Kumamoto - again, the train is faster than the bus, but both are infrequent.
Kumamoto is nice for its castle, plus don't miss the beautiful Suizenji Garden:
https://www.youtube.com/watch?v=K9xyXmU8U1g

I'm traveling by train. Preferably locals as I can't get rail passes.
How long will it take to visit aso itself?

You can take the Kyusanko Odan bus which departs Beppu stn at 8:09, arrives Aso stn at 11:23. From there take a local bus to Asozan stn, where you take ropeway/microbus to crater. You can continue on to Kumamoto from Aso stn by either bus or local train. Local train requires transfer at Higoozu.